There's a sound mod available for MBT. It was originally developed for WinSPWW2, but as MBT uses the same sound map it works just as well with MBT. (As long as your install is using the default WAV sounds not MP3)
You can read more about it here:
http://www.theblitz.org/message_boar....php?tid=46677
This mod only changes the wav files (actual sounds), so can be installed mid-battle with no PBEM, security or OOB problems.
However, this mod is not endorsed or supported by Shrapnel Games or The Camo Workshop. Here's their advisory on mods:
"This mod contains highly altered or additional files that may or will cause interference with the master game files and it is HIGHLY RECOMMENDED that this mod be installed and played ONLY in a separate copy of the game and NOT run from the master game"
Sound Mod for WinSPMBT
Threaded Mode